In a conventional multicarrier transmission system, transceivers communicate over a communication channel using multicarrier modulation or Discrete Multitone Modulation (DMT). Carriers or subchannels spaced within a usable frequency band of the communication channel are modulated at a symbol transmission rate of the system. In ADSL (Asynchronous Digital Subscriber Line) systems, the symbol rate is approximately 4 kHz. Every 250 microseconds, the transmitting transceiver allocates a new set of bits for transmission to the subchannels so that the bit error rates of the subchannels are substantially equal at the receiving transceiver. Consequently, for a given symbol period the numbers of bits may vary from subchannel to subchannel.
ITU (International Telecommunication Union) standards G.992.1 and G.992.2 specify parameters that characterize the operation of ADSL DMT transceivers. Examples of parameters, to name but a few, include the data rate (b/s) for the connection between the transceivers, the number of subchannels in the upstream and downstream directions, and the number of bits allocated to each subchannel. In general, such parameters remain fixed after the initial configuration and installation of the transceiver. Some parameters depend on the data rate of the connection and may change when the ADSL connection is at a high or low data rate. Some parameters may change when channel conditions change. However, after the type of application is determined, i.e., voice, data, video, etc., the parameters are optimized and fixed for that application.
For splitterless operation, described in the ITU G.922.2 standard, ADSL transceivers store “channel profiles,” which include a subset of parameters that are used when conditions of the communication channel change (e.g., a telephone goes off the hook). When the channel conditions change because of an event that the ADSL transceiver does not control (e.g., a telephone connected to the same wire as the ADSL transceiver goes off hook), the ADSL transceiver must identify the new channel condition, retrain some of the receiver functions (e.g., equalizers, echo cancellers, etc.), and switch to the channel profile that is used for the new channel condition. This process, defined as a “Fast Retrain” procedure in ITU G.922.2, takes approximately 1-2 seconds. These channel profiles, however, depend solely on the channel condition and not on the application(s) executing on the ADSL connection.
As technological advances increase the data rate throughput for multicarrier transmission systems, ADSL transceivers are becoming capable of supporting multiple applications. To support multiple applications, it is necessary that the ADSL transceiver be able to quickly and efficiently adapt the transmission parameters as the number and type of active applications change over time. For example, if the ADSL transceiver is accessing data over the Internet when a voice telephone call that is being transported over the ADSL connection becomes active, the ADSL transceiver must be able to modify the transmission parameters to accommodate both active applications. As other voice telephone calls and different applications (e.g.,. video on demand, video conferencing) become activated and deactivated over the ADSL connection, the ADSL transceiver must also be able to support the various transmission requirements of the various combinations of concurrently active applications. For example, video signals have higher reliability but lesser transmission delay requirements than voice and data signals. For some transmission systems, it has been necessary to find a compromise between high reliability and transmission delay.
Thus, there remains a need for a system and method that can support the various transmission requirements of multiple active applications as the number and type of active applications change over time.